1. Field of the Invention
The present invention relates to a heat insulating window film, a heat insulating window glass, a building material, a window, a building, and a vehicle. More specifically, the invention relates to a heat insulating window film, a heat insulating window glass, an building material, a window, a building, and a vehicle which are manufactured at low cost, easily manufactured to have a large area, and have excellent visible light transmittance, shielding properties for near infrared light, heat insulating properties, and light resistance.
2. Description of the Related Art
In recent years, products with a lower environmental burden, which are so-called eco-friendly products have been required as one of energy saving measures for carbon dioxide reduction, and solar control window films for windows of vehicles or buildings have been required. A heat insulating window film is considered as such a product. The heat insulating window film is a film which delays transmission and reception of heat between an indoor side and an outdoor side by being attached to windows, and usage of heating and cooling is reduced by using this film, and therefore, energy saving can be expected. A degree of heat insulation is defined by using a coefficient of overall heat transmission. In the solar control window film procurement standard in the Law Concerning the Promotion of Procurement of Eco-Friendly Goods and Services by the State and Other Entities (so-called Green Purchasing Law), heat insulating properties are determined to be obtained when a coefficient of overall heat transmission is less than 5.9 W(m2·K) measured by using a measurement method based on JIS A 5759 “Adhesive films for glazing” (JIS stands for Japanese Industrial Standards). When the numerical value thereof is small, the transmission of heat is delayed and heat insulating properties are increased. According to JIS A 5759, a coefficient of overall heat transmission can be acquired from reflection spectra of far infrared rays at a wavelength of 5 μm to 50 μm. That is, it is preferable to increase reflectivity of far infrared rays at a wavelength of 5 μm to 50 μm, in order to decrease a coefficient of overall heat transmission.
In the solar control window film procurement standard in the Green Purchasing Law, it is necessary to decrease a shade factor, in order to increase a quantity of heat flow for sunlight. As a method of decreasing a shade factor, a method of using a near infrared shielding material capable of increasing shielding properties of near infrared light is used.
H. A. Macleod. Thin-Film Optical Filters, The Nikkan Kogyo Shimbun, 1989, p 599-602 discloses that a layer reflecting infrared rays can be prepared by interposing a metal layer between two dielectric layers, as the method of increasing the reflectivity of infrared rays.